1. Field of the Invention
The present invention relates generally to cell selection, and more particularly, to cell reselection of a multi-mode mobile terminal from a legacy network to an advanced network in a mobile communication system.
2. Description of the Related Art
Upon being powered-on, a Mobile Terminal (MT) performs a cell selection procedure to determine a suitable cell that may be used for camping. Hereinafter, camping refers to the process in which the MT chooses the suitable cell and tunes to the control channel of the cell. Cell reselection is generally performed when the MT finds a more suitable cell for camping than the previously selected cell.
The cell selection and reselection procedures for an MT supporting 3rd Generation Partnership Project (3GPP) Release 8 are defined as shown in FIG. 1, which illustrates the 3GPP Legacy and 3GPP Long Term Evolution (LTE) network deployment according to the prior art. Hereinafter, a legacy network refers to a previous generation standard network, including 2G Global System for Mobile communication (GSM)/Enhanced Data rates for Global Evolution (EDGE), 3GPP2 Code Division Multiple Access (CDMA), and 3G Wideband CDMA network, and these terms may be used interchangeably herein. An advanced network as described herein includes a 4G LTE network and a 4G Advanced LTE network. These terms may also be used interchangeably herein.
Referring to FIG. 1, an MT 10 is located in an area of a wireless communication system including a Base Station (BS) 20, an evolved Universal Terrestrial Radio Access Network (eUTRAN) 30, an evolved Node B (eNB) 40 and a plurality of Node Bs (NB1, NB2, NB3). The MT herein refers to a multi-mode terminal that supports both the legacy network and the advanced network. As shown in FIG. 1, the cell area of the eNB 40 overlaps the cell areas of the BS 20, NB1 and NB3. The eNB 40 provides a packet radio base station for the eUTRAN 30.
FIG. 2 illustrates a cell reselection from a legacy network to an advanced (LTE) network according to the prior art. Referring to FIG. 2, in step 201 a terminal is in an idle state in the legacy network (2G/3G) when measurement conditions are met. In step 202, an LTE system measurement is requested by the legacy terminal 50, and after the measurement procedure is performed in step 203, a measurement response is received at the legacy terminal 50 from the LTE terminal 60 in step 204. When it is determined by the legacy terminal 50 to perform a cell reselection in step 205, an Inter Radio Access Technology (I-RAT) cell reselection request in step 206 is made to the LTE terminal 60, which selects an eNB 40 of a cell to which the reselection will be performed in step 207. The cell reselection procedure is then performed in step 208, and in step 209 an I-RAT cell reselection confirmation is sent from the LTE terminal 60 to the legacy terminal 50.
FIG. 3 illustrates the opposite procedure of FIG. 2, i.e., a cell reselection from an LTE network to a legacy network according to the prior art. Referring to FIG. 3, in step 301 a terminal is in an idle state in the advanced network (LTE) 60 when measurement conditions are met. In step 302, legacy system measurement is requested from the LTE terminal 60. After the measurement procedure is performed over the legacy terminal 50, an eNB 40 and a legacy network 70 in step 303, and a measurement response from the legacy terminal 50 is received at the LTE terminal 60 in step 304. When it is determined to perform a cell reselection from the LTE terminal 60 to the legacy terminal 50 in step 305, a cell reselection request in step 306 is made to the legacy terminal 50, and the cell selection procedure is performed over the legacy terminal 50, the eNB 40 and a legacy network 70 of a cell to which the reselection will be performed in step 307. In step 308, an I-RAT cell reselection confirmation is sent from the legacy terminal 50 to the LTE terminal 60.
The migration of a BS in a wireless network from a legacy network to an advanced network is a time-consuming process. The advanced networks, i.e., 4G LTE and 4G Advanced LTE, can support cell selection and cell reselection of an MT by a 2G GSM/EDGE or 3G WCMDA system supporting a previous version 3GPP standard, since the 4G systems have the capability to measure power of the BS in the legacy network. This capability is realized by the acquisition of information of a neighbor cell that supports an advanced network.
In contrast, a legacy network cannot support cell selection and cell reselection of an MT by a system supporting a 4G version 3GPP standard, because the legacy network cannot acquire information of the neighbor cell that supports an advanced network.
In the 3GPP standard, a measurement configuration message including information is received through Layer 3, and the information is used to perform a measurement of another network. In certain instances, a BS supporting a legacy network cannot transmit this information to the MT, preventing the MT from finding an advanced network. A periodic search is thus performed by the MT, to find a suitable LTE network for camping.
In those instances when the BS can transmit the information of the measurement configuration message to the MT, it is possible for the MT to recognize the LTE cell in the neighborhood. However, this recognition is not possible in the legacy networks by virtue of transmission of this information from the BS, which does not support the advanced network, such as 3GPP Releases 7, 8 or later, at the early deployment of the advanced network. Moreover, the MT in the legacy network may attempt to search for cells that cannot be selected since there are no cells for the LTE network, thereby incurring a waste in power consumption.